When you begin building your level start simple. I usually do not begin by
building one room in a level and then adding every single little detail and
then moving on to the next. Instead I usually build a shell that represents
that room and then move on creating halls and other rooms as I go. Once I get
the basic flow of the level laid out in WED I then begin to add details to
each section. The types of details I add will depend on the purpose and function
of the area.

For example, take a look at the first set of halls in second level of The
Test:

I first laid out a pretty basic hall. The walls very very
straight and at 90 degree angles like most hallways. The floor was nice and
flat. All in all, it was pretty boring. So I decided it need to be made more
interesting. The first thing I did was decide the walls should not be at 90
degree angles. I angled them slightly inward:

I then decided the floor was too typical and so I created an
indented area covered with a metal floor grating:

Lastly I added support structures to the wall to sort of break
up the hall a little. I did this because I knew it would add nice shadows when
lights were added later on:

Since we are looking at halls, let's look at the hall in the
second part of the same level:

This hall is much different from the first one. There are two
reasons for this. The first reason is that this is a different area of the
level. The second is I simply wanted a different look so as not to bore the
player. Use your creativity to make unique and interesting areas, but at the
same time keep in mind the overall purpose of the areas you are creating.